This invention relates to an integrated hydrofoil suspension and steering system in a hydrofoil boat utilizing many other individual features in combination to improve safety, simplicity and economy of operation and of construction. More particularly, the invention relates to a hydrofoil suspension and steering system producing coordinated, aircraft type turns. The preferred embodiment includes one or more pairs of foils arranged on opposite sides of the longitudinal axis of the boat which may be controlled simultaneously to convert from a submerged to a surface piercing configuration. Starboard and port foils may also be controlled independently in conjunction with the production of a proportional yawing moment to produce coordinated turns with a single control movement.
Prior art hydrofoil boats have utilized fixed foils in submerged or in surface piercing configuration, and certain advantages accrue to both methods of suspension. Submerged foils produce more lift at lower speed than a surface piercing foil of the same size because of its larger wetted area and the more vertical orientation of the lift vector. However, the lift vector of the surface piercing foil is generally inclined, as viewed from the front, toward the vehicle center of gravity, making the surface piercing foil the more stable configuration. Stability is a factor of some importance, since all hydrofoils are top heavy when foil borne.
In a smaller boat, while it is desirable to become foil borne at as low a speed as possible and to operate the foils when hull borne in a shallow draft or beaching mode, stability must also be considered. Crossing the wakes of other boats, manuevering, and operating at angles to significant wave action are a few examples of operating regimes where stability is desired. The present invention allows the operator of the craft to select the most advantageous mode of operation.
Submerged foils suffer interference drag losses at the intersection of the foil and the supporting strut. At higher speeds, the surface piercing mode of operation is far more efficient, enabling operation at higher speeds and with greater economy than at the submerged mode, since this form of drag can be eliminated. Thus the present invention permits both optimum low and high speed modes of operation.
At constant speed, prior art boats control attitude, height above the surface and trim by control tabs on the foils or by changing foil angle of attack. At constant speed, changing from submerged to surface piercing operation and back, as taught by this invention, raises and lowers the boat in the water and, by operating the foils independently, attitude and trim of the craft can be controlled.
A few prior art boats have utilized differential angles of inclination of hydrofoils to bank the boat. When the foils on one side of the boat are inclined away from the horizontal less vertical lift is produced and the boat lists toward the more vertical foil. However, this maneuver and maneuvers executed by the more conventional (no bank control) hydrofoils, produces a momentary side force opposite to the direction of the turn, the horizontal component of the lift vector of the more vertically inclined foil. Side ventilation of the struts in the submerged mode or foils in the surface-piercing mode can occur during these uncoordinated turns causing increased drag and stress. The subject invention includes the input of a yawing moment about the vertical axis to oppose this side force; avoid side ventilation, reducing drag and stress; and this yaw may also be used to tighten the turn.
Since the present invention utilizes pairs of foils which may be independently inclined from approximately horizontal or parallel to the plane of the lateral and longitudinal axes of the boat to surface-piercing positions at an angle to this plane many factors may be controlled by the operator including draft of the boat at low speed, height above the surface at any given foil borne speed, bank angle for turns, trim and attitude about both lateral and longitudinal axes and stability. Changing the sweep back angle differentially between pairs of foils also permits banking without creating a momentary sideforce opposite to the direction of banking and turning insofar as the lift vectors of the foils are concerned. The less highly swept foil may, depending upon the banked, wetted area of each foil, have a higher drag which will cause yaw without a counteracting yawing moment.
Prior art hydrofoil boats have used conventional rudders to effect turns, incurring a drag penalty during use. This penalty can be significant during a long run with a strong cross wind or current. The present invention includes a rear, symmetrical wing foil of positive dihedral which may be rotated to produce a yawing moment with no loss of total foil lift and no increase in drag.
At higher speeds associated with hydrofoil boats, collisions between the foils and submerged objects are of great concern. Prior art hydrofoils include hinged struts and other methods of releasably rotating the foil support structure, should the foil strike a submerged object. Unfortunately, the prior art does not provide for safe transition to hull borne operation and the danger of cartwheeling or otherwise capsizing is high; particularly in a smaller craft at high speed. Should any main foil of the subject boat strike a submerged object; shearing fasteners fail, allowing the foil to swivel and streamline, converting to a ski; thus permitting a safe, stable deceleration of the craft.
Other features of the boat include modular construction using composite and other non-corrosive, lightweight materials, ducted fan propulsion driven by synchronous belt drive, convertibility to a wheeled trailer and a simple, interconnected control system.
Although the utility and pleasure boat industries are thriving in these days of increased leisure activity and spending for recreational equipment, the hydrofoil runabout or sport craft is conspicuous in its absence from a plethora of other types. In fact, hydrofoils are rarely found among smaller class utility craft. This is even more unusual in the light of the public's desire for increased speed in pleasure craft as evidenced by larger engines, planing hulls and other means of increasing the speed of pleasure craft. The hydrofoil boat is an obvious next step because of its speed and efficiency but no such pleasure or utility craft appear to be in quantity production. It is suggested that this lack is due to the failure of prior art designs to incorporate safety, simplicity and affordability or economy of construction in proper combination.
The instant design makes the hydrofoil boat practical and attractive through a combination of many improvements as described above and by providing aircraft type control; quiet, comfortable operation despite various water surface conditions; simple, practical construction; economical propulsion even at high speeds; safe operation at those high speeds despite the danger of submerged objects, and other practical features such as convertability for self-trailering.